Sand milling process and apparatus



W. HUGHES ETAL SAND MILLING PROCESS AND APPARATUS A A Ma -2s; 1965 Filed Jul so. 1962 \NNN SAND MILLIN G rnocnss AND APPARATUS William Hughes, Norton-on-Tees, and Arthur Dolby Brown, Grimsby, England, assignors to .British Titan Products Company Limited, Billingham, County Durham, England, a corporationof GreatBritain Filed July 30, 1962, Ser. No. 213,535 Claims priority, application Great Britain, Aug. 3, 1961,

20 Claims. (Cl. 241-20) The present invention relates to a process for the grinding of particulate solids in. liquids. The invention also relates to an apparatus for carrying out such a process. Since grinding processes and apparatus can be used for dispersing a solid in a liquid without any appreciable reduction in the size of the solid particles necessarily occurring, the term grinding is herein used to include dispersing? Processes for the grinding of particulate solids, in which the solid is agitated with a liquid and a grinding medium such as sand, have been previously described, for example British specification No. 679,552. Processes have also been described for the dispersion of finely divided pigments in film forming liquids.

One particularly suitable form of apparatus in which such grinding can be accomplished comprises a vertical container for holding the solid to be ground or dispersed, the grinding medium, and a liquid. The mixture is then agitated by meansof impellers affixed to a rotating shaft immersed in the mixture.

. Such apparatus can be operated continuously by supplying the particulate solid and liquid, for example as a slurry, to the container which contains the grinding medium and thereafter removing from the container the ground solid and liquid as a suspension. The grinding medium in such apparatus is separated from the solid/ liquid slurry before the latter leaves the machine by means of a wire sieve screen of the appropriate mesh size.

Difficulties have been encountered with such apparatus, particularly when continuously operated at high levels of throughput of solids to beground. Such difficulties have beendue to blocking of the screen, the mesh size of which is necessarily smaller than the mean particle size of the grinding medium; This necessitates reduced throughput of material and/or frequent stopping of the apparatus to clear the sieve.

It is an object of the present invention to provide an improved process for the-grinding of solids in liquids. It

is also an object of theinvention to provide an improved apparatus for carrying out such grinding.

Accordingly, the .processof the present invention comprises supplying solid particles to be ground and a liquid to a first zone containing agitated particulate grinding medium; passing the slurry thus formed into a second zone where grinding medium is allowed to settle out preferentially tothe ground'solid particles; and thereafter removing from the second zone said liquid and the ground solid particles free from at least most of the grindingmedium. The grinding medium can then be returned to the first zone, with or without any of the ground solid which may have. settled out with the grinding medium.

In order to achieve the preferential settling of the grinding medium, the settling rate of the grinding medium should of course be greater than the settling rate of the ground solid. This settling rate depends both on the density and on the particle size of the material in question.

In the preferred form of the invention, the second zone is constituted by a vertical container and the slurry from the first or grinding zone is fedinto the lower portion of such container, while said liquid and the ground solid particles are recovered from the upper portion of such container. The vertical rate of flow of the slurry upwards in the container is regulated so as not to entrain most of the grinding medium but to allow the latter to settle towards the bottom of the container. From there the grinding medium can be returned to the first or grinding zone.

It is preferable to ensure that the slurry flows smoothly into and upwardly through the said vertical container, thus avoiding turbulent flow which would inhibit rapid and eflicient settling of the grinding medium out of the slurry. This may conveniently be achieved by constructing the vertical chamber so that the area of its horizontal cross-section is greater in its upper part than in its lower part; most desirably, such area increases continuously from near the lower end of the vertical container to near its upper end so that the vertical velocity of the slurry is reduced as it passes upwards. The vertical cross-sec ftion of said container is suitably in the form of an inverted frusto-cone.

Alternatively, the second zone may have a horizontal cross-sectional area no greater than that of the first zone, provided of course that the grinding medium is able to settle out preferentially as described above. Such second zone may particularly be used when a heavy grinding medium is used. However, such second zone is not th preferred second zone of the invention.

If the horizontal cross-section of the vertical container is, at least in the upper part of such container, greater than that of the grinding chamber constituting the first zone, one may be able to maintain a high velocity of flow in the first zone without making the rate of flow through .the vertical container too great for efficient separation of the grinding medium from the slurry.

Said container is advantageously located immediately above the outlet of thegrinding chamber constituting the first or grinding zone. A neat continuous process can then be achieved, the ground slurry continuously passing upwards into the settling container of the second zone and the grinding medium continuously falling downwards from the second zone container back into the first zone grinding chamber. In fact, the second zone container may, in effect, he a vertical continuation of the outlet end of the first zone grinding chamber.

It will be seen, therefore, that the invention also provides an apparatus for grinding solid in liquid comprising interconnected first and second chambers (the horizontal cross sectional area of the second chamber preferably being, at least in the upper part of said second zone, greater than that of the first zone), and means for:

(a) Agitating the contents of the first chamber,

(b) Passing said'contents to the second chamber,

.(c) Efiecting upward movement of contents in the second chamber, and (d) Collecting a solid-liquid slurry from the upper part of the second chamber.

The first and second chambers are conveniently lower and upper sections of a vessel respectively. At least the upper part of the upper section will then preferably have a larger cross sectional area than the lower section. It is preferred that the upper section have a wall outwardly inclined so that the upper section, or a part thereof, should be in the form of an inverted frusto-cone placed upon the lower section. The lower section is preferably cylindrical. A

When the process is operated continuously, that is by the continuous addition of solid and liquid, the slurry rising from the lower section into the upper section therefore rises at a slower rate in the upper section due to the larger horizontal cross section of the latter; This cross section is so chosen that the rate of rise of the slurry is less than the settling rate of the grinding medium in the slurry, thus causing a separation between the grinding medium and the solid/liquid slurry. The separation of the grinding medium from the solid/liquid slurry is assisted by a lack of turbulence in the upper section.

When an inverted frusto-conical upper section is used, the grinding medium settles out on the inclined wall of the upper section and is returned by gravity to the lower section for further use.

The means for agitating the slurry of grinding medium, solid and liquid in the first chambers is preferably a shaft extending into this chamber carrying impellers and caused to rotate by an electric motor.

The impellers upon the shaft are conveniently discs fixed to the shaft and concentric with its longitudinal axis. The discs are suitably of such diameter that only a rela tively small space exists between the periphery of the discs and the walls of the first zone, so that the solid to be ground should not readily pass through the first chamber thus avoiding the action of the discs and the consequent grinding of the solid. For example, the distance between the periphery of each disc and the nearest wall may be about /2 to 1 /2 inches, although this distance is not critical and may be varied as required.

Similarly, the distance apart of the impellers is not critical. The optimum distance Will depend upon various factors and this can readily be discovered by experiment in any particular case. It is often found that such optimum distance is in the range 1-10 inches.

In the case of an upright cylindrical lower chamber into which the shaft extends along the axis of the cylinder, the distance between the lowest impeller and the bottom of the cylinder may vary widely. For example, I

such distance may often suitably be about the same as the distance apart of the impellers. I

The impellers may have projections on their upper or lower surfaces, or both. They may be formed from, or coated with a polyurethane rubber.

The means for collecting solid and liquid slurry (after at least the greater part of the grinding medium has settled out) is conveniently a channel surrounding the upper edge of the mouth of the. second chamber, particularly when an upper section of an inverted frusto-cone shape is used as a second chamber. From this channel the solid/liquid slurry may be led away by collecting pipes in the floor of the channel for further treatment as desired.

The solid to be ground is preferably supplied to the bottom of the first zone. For example, a slurry containing such solid may be fed to the base of the lower section of the vessel. The ground or dispersed solid may be recovered in a solid/liquid slurry from the top of the second zone, ensuring that the solid passes through the whole of the first zone where the grinding mainly takes place. It has been found advantageous to place the lower impeller opposite the slurry inlet and to provide the lower surface of such lower impeller with blades after the fashion of a propeller. By this means a rotating motion is immediately imparted to the incoming slurry, and wear upon the surface of the impellers may thus be reduced.

Examples of a solid to be ground are a pigment which requires further grinding to improve its tinting strength, and/or texture, and a pigment which is to be dispersed in a liquid. In the former case the liquid added to the vessel may be any desired liquid, for example water. In the latter case the liquid is of course that in which it is desired to disperse the solid, for example a film forming liquid such as a paint base.

The concentration of the solid in the slurry fed into the first zone may vary. For example, the concentration of titanium dioxide pigments being ground in water is suitably 200l,000, preferably not more than 600, grams/litre.

Where titanium dioxide pigment is to be ground to improve its tinting strength, it is preferably ground after having been calcined and dry milled.

The grinding medium should be in a relatively finelydivided state to enable it to be agitated satisfactorily by the impellers, although its mean particle size should be greater than those of the ground or dispersed solid, so as to facilitate efficient separation of the grinding medium from the solid/liquid slurry in the second zone. Materials such as glass ballotini, silica, zircon alumina or titanium dioxide have been found very satisfactory as grinding media. When titanium dioxide is being ground, the grinding medium generally should have a mean particle size of microns to 3,000 microns, preferably 250 microns to 2,500 microns.

When the impellers are driven at high speeds, it has been found that a vortex maybe formed at the junction of the first and second zones, and this is undesirable, since it hinders deposition of the grinding medium in the second zone. The formation of a vortex may be avoided by the provision of baffles or the like where the vortex forms, to break up the swirling motion of the mixture at this point. One convenient method of providing such baffles is to fix them to a collar which surrounds, but is not atached to, the rotating shaft driving the impellers.

It has also been found advantageous, when the first and second zones consist respectively of a cylindrical lower section and an inverted frusto-conical upper section superposed thereon, to place initially in the apparatus a volume of grinding medium which is in excess of the volume left unoccupied in the lower section by the impellers and shaft; that is, it has been found advantageous to carry out the process in the presence of sufiicient grinding medium, not only to fill the lower section with the shaft and impellers in position, but to fill also part of the upper section so that such grinding medium should extend some distance up into this upper section. It is believed that the presence of such a volume of grinding medium increases the pressure upon the mixture surrounding the impellers and restricts to some extent the movement of the mixture away from the vicinity of the impellers, where it is believed that the greater part of the grinding takes place. The concentration of the grinding medium around the impellers, is also increased.

The time taken to grind a solid will depend on the design of the apparatus and on the nature of the solid and, when the solid is to be dispersed in a liquid, may depend also on the nature of the liquid. In the case of titanium dioxide pigments which are to be ground in water in an effieient apparatus, it has been found generally that a residence time of from 1 to 6 minutes, preferably from 5 to 25 minutes, gives a product of good tinting strength and brightness. Indeed, little improvement is found normally with residence times greater than about 40 minutes.

The grinding of some materials may be affected by the temperature at which the grinding is carried out. If flocculation, for example, is caused by relatively high temperatures, it is a simple matter to cool the grinding machine (for example by the provision of a cooling jacket around the vessel).

It has been found advantageous to introduce a liquid, e.g. water, into the second zone in order to dilute the slurry and thus to facilitate the separation of grinding medium and solid/liquid slurry by reducing the viscosity of the mixture. In the preferred embodiment the liquid is introduced through the sides of the inverted frustoconical upper section and serves to dilute the slurry in this section.

It has been found in practice that the process of the present invention is often capable of retaining at least 99%, of the grinding medium particles from the slurry which passes from the first or grinding zone to the second zone, and that the amount of grinding medium particles passing from the second zone may generally be a few parts per million.

The invention is illustrated by the accompanying drawing which is a partial cross-sectional view of lower and upper sections. I

The following example illustrates the present invention.

Example 1 The appa-ratus shown in FIG. 1 was set It comameter, carrying five polyurethane rubber impellers,

each 17 /2 in diameter and 1 /2 thick. The lower impeller 5 was provided with dependent blades 5a and was positioned 4" above the base of the vessel 1, and the remaining impellers 6were in the form of discs and were supported 4 apart by means of spacers 7 5 /2" in outsidddiameter. The blades 5a acted as a type of propeller and gave -a rotary motion to the slurry in the vessel.

The shaft was supported in bearings (not shown) and was driven through a belt by a 47.5 HP. electric motor (not shown) Around the outer rim of the upper section of the vessel 2 there was provided a collecting channel 8 from which led collecting pipes 9.

The portion of the shaft in theupper section of the vessel was surrounded by a collar 10 carrying four vertical bafiies 11, each 11 in height, to minimise vortex tormation in the upper section of the vessel.

Four water inlets 12 were provided in the sides of the upper section of the vessel. These were connected by 1" internal diameter pipes 13toa water supply (not shown).

In the operation of the apparatus, a charge of 2,200 lbs. of Ottawa sand (particle size: 75.8% .in the range -16 +22 British Standard Sieve mesh; 23.8% in the range 22 +30 British Standard Sieve mesh; the remainder in the range 30 +44 British Standard Sieve mesh) was placed in the vessel and the shaft 4 was rotated at a speed of 560 rpm. (peripheral speed of the impellers: 2,570 ft./ min).

Calcined titanium dioxide slurried in water at a concentration of about 300900 grams/ litre was then supplied by the gravity feed through the inlet 3 into the lower section 1 at rates between about 0.516 and about 1.8 cu. metres per hour. These concentrations and rates represented throughputs of pigment of from 10.8 to 39 tons per day at residence times in the vessel of from 14.8 to 4.2 minutes. 7

Additional water was supplied through the inlets 12 to adjust the slurry concentration in the upper section 2 to about 450 grams/litre (the amount of water added varied between 0.3 and 1.5 cu. metres per hour).

As the titanium dioxide/ water slurry was fed through the inlet 3 into the base of the lower section of the vessel 1, the liquid rose in the conical upper section 2 and overflowed into the collecting channel 8. No vortices formed around the upper pontion of the impeller shaft, and separation of the sand from the ground solid/water slurry was excellent (the material collecting in the channel being substantially free from sand and believed in fact to contain less than 10- gramis/ litre of sand).

Example 2 The apparatus used was similar to that of Example 1. The shaft was rotated at 625 r.p.m., giving a peripheral speed of 2870 feet per minute.

having a concentration of 213 grams per litre. The average residence time of the titanium dioxide in the grinding zone was 39.8 minutes Example The apparatus used was similar to that of Example 1.

A titanium dioxide slurry containing 438 gramsper litre of titanium dioxide was fed to the apparatus at a rate of 0.36 cubic metre per hour. The titanium dioxide slurry fed contained 1.65% sodium silicate (as SiO on the titanium dioxide. Sufficient water was added to the second zone to give a product containing 334 grams per litre and the average residence time was 21.2 minutes.

The amount of sand in the ground solid/water slurry recovered from the processes of Examples 2 and 3 was similar to that in the case of Example 1.

In addition to the ready separation of the grinding medium from the solid/water slurry, the ground material in all three examples increased in tinting strength,

improved in texture, and had excellent brightness.

What is claimed is:

1. A process for the grinding of solids in liquid, comprising supplying solid particles to be ground and a liquid to a grinding zone containing agitated particulate grinding medium; passing the resulting slurry of solid particles to be ground and particulate grinding medium with substantially unimpeded flow upward through a separating zone; reducing the vertical velocity of said slurry prior to the outflow of solid-liquid slurry' from said separating zone to a value substantially lessthan the velocity of said slurry in said grinding zone and substantially less than the suspending velocity of said grinding medium but above the suspending velocity of the ground solid particles, whereby entrained particles of grinding medium setle out of said slurry in preference to the ground solid particles; and thereafter removing from the upper portion of said separating zone said liquid and the ground solid particles free from at least most of the grindingmedium.

2. A process according to claim 1 in which the grinding medium is thereafter returned from the separating zone to the grinding zone. 1

3. A process according to claim 1 in which the sepaground and particulate grinding medium continuously passing upwards into the separating zone and the grinding medium continuously falling downwards from the separating zone back into the grinding zone.

4. A process according to claim 3 in which the volume of grinding medium is in excess of the 'free volume of the first zone.

5. A process according to claim 3 in which a liquid is introduced into the separating zone separately from the slurry from the grinding zone, the liquid serving to dilute the slurry in the separating zone so as .to facilitate the separation of the mixture of grinding medium and solid-liquid slurry by reducing the viscosity of the mixture.

6. A process according to claim 5 in which the liquid is water.

7. A process for the grinding of solids in liquid, comprising the steps of (a) providing an agitated grinding zone containing a particulate grinding medium;

(b) introducing solid particles to be ground and a liquid into said agitated grinding zone, said solid particles being brought into grinding contact with said particulate grinding medium;

(c) maintaining said solid particles and said particulate grinding medium in said grinding contact to form a slurry in said liquid of said solid particles wherein said solid particles are of substantially reduced particle size;

(d) removing said slurry together with entrained particulate grinding medium from said grinding zone;

(e) introducing said slurry with substantially unimpeded flow into a settling zone in a manner to cause said slurry to pass through said settling zone in a generally vertically upward direction;

(1) reducing the vertical velocity of said slurry prior to the substantial outflow of said slurry to a value below its velocity in said grinding zone, and below the suspending velocity of said particulate grinding medium but above the suspending velocity of said solid particles of substantially reduced particle size, whereby particles of said entrained grinding medium settle from said slurry;

(g) recovering settled particulate grinding medium from the lower region of said settling zone and (h) recovering .slurry substantially free from particulate grinding medium from the upper region of said settling zone.

8. Apparatus for the grinding of solids in liquids comprising a first chamber having an inlet and an exit; means for agitating the contents of said first chamber; a second chamber having an inlet and an exit, the horizontal cross-sectional area of at least the upper part of said chamber being greater than that of said first chamber, said second chamber being disposed with its said exit above its said inlet, the walls defining at least the lower portion of said chamber and including at least a substantial part of the chamber of greater horizontal crosssectional area being substantially impermeable to the outflow of liquids substantially unimpeded conduit means connecting said exit of said first chamber and said inlet of said second chamber; means for passing a substantial portion of the contents of said first chamber through said conduit means into said second chamber and for eifecting the upward movement of materials in said second chamber; and means above said part of said second chamber of greater cross-sectional area for collecting materials from the upper part of said second chamber.

9. Apparatus for the grinding of solids in liquids comprising a first cylindrical chamber having an inlet in the lower portion thereof and an exit in the upper portion thereof; a second chamber having a general form of an inverted frusto-cone and having an inlet in the lower portion thereof and an exit in the upper portion thereof; slurry conduit means connecting said exit of said first chamber and said inlet of said second chamber; means for passing slurry from said first chamber through said slurry conduit means into said second chamber and for effecting the upward movement of slurry in said second chamber; and means for collecting slurry from the upper part of said second chamber.

10. An apparatus according to claim 9 in which the first and second chambers are, respectively, lower and upper sections of a single vessel, the conduit means connecting said chamber being formed by the super position of said inlet of said second chamber on the exit of said first chamber.

11. Apparatus according to claim 8 in which the second chamber is disposed vertically above the first chamber.

'12. Apparatus according to claim 11 in which the first and second chambers are lower and upper sections of a vessel, respectively, the conduit means connecting said chambers being formed by the super position of said inlet of said chamber on the exit of said first chamber.

-13. Apparatus according to claim 8 in which the second chamber has a wall outwardly inclined.

14. Apparatus according to claim 8 in which the second chamber has the form of an inverted frusto-cone.

15. Apparatus according to claim 10 in which the means for agitating the contents of the first chamber comprises a rotatable shaft extending into this chamber carrying impellers in this chamber.

16. Apparatus according to claim 15 in which the impellers are discs fixed to the shaft and concentric with its longitudinal axis.

17. Apparatus according to claim 16 in which the discs are of such diameter that only a relatively small space exists between the periphery of the discs and the walls of the first chamber.

18. Apparatus according to claim 17 in which the distance between the periphery of each disc and the nearest wall is 0.5-1.5 inches.

19. Apparatus according to claim 18 in which the distance apart of the impellers is 1-10 inches.

20. Apparatus according to claim 19 in which the means for collecting a solid-liquid slurry from the upper part of the second chamber comprises a channel surrounding the upper edge of the mouth of the second chamber.

References Cited by the Examiner UNITED STATES PATENTS 1,956,293 4/34 Klein et a1.

2,104,709 'l/ 3 8 Weinig 241-20 2,212,260 8/40 Brothman.

2,779,752 1/57 Vining.

2,855,156 10/58 Hochberg et al 24122 3,075,710 1/63 Feld et a1 24122 X 3,134,549 5/64 Quackenbush 241-172 X J. SPENCER OVERHOLSER, Primary Examiner. 

1. PROCESS FOR THE GRINDING OF SOLIDS IN LIQUID, COMPRISING SUPPLYING SOLID PARTICLES TO BE GROUND AND A LIQUID TO A GRINDING ZONE CONTAINING AGITATED PARTICULATE GRINDING MEDIUM; PASSING THE RESULTING SLURRY MEDIUM WITH TICLES TO BE GROUND AND PARTICULATE GRINDING MEDIUM WITH SUBSTANTIALLY UNIMPEDED FLOW UPWARD THROUGH A SEPARATING ZONE; REDUCING THE VERTICAL VELOCITY OF SAID SLURRY PRIOR TO THE OUTFLOW OF SOLID-LIQUID SLURRY FROM SAID SEPARATING ZONE TO A VALUE SUBSTANTIALLY LESS THAN THE VELOCITY OF SAID SLURRY IN SAID GRINDING ZONE AND SUBSTANTIALLY LESS THAN THE SUSPENDING VELOCITY OF SAID GRINDING MEDIUM BUT ABOVE THE SUSPENDING VELOCITY OF THE GROUND SOLID PARTICLES, WHEREBY ENTRAINED PARTICLES OF GRINDING MEDIUM SETLE OUT OF SAID SLURRY IN PREFERENCE TO THE GROUND SOLID PARTICLES; AND THEREAFTER REMOVING FROM THE UPPER PORTION OF SAID SEPARATING ZONE SAID LIQUID AND THE GROUND SOLID PARTICLES FREE FROM AT LEAST MOST OF THE GRINDING MEDIUM. 